(a) Field of the Invention
The present invention concerns a process and a device for freezing articles, such as foods, which are in solid, pasty or liquid form.
(b) Description of Prior Art
The freezing of an article by complete or incomplete immersion in a bath of a cryogenic liquid such as liquid nitrogen, is well known. Thus, patent application GB 2,092,880 describes a process for freezing articles which are liquid, for example blood, or pasty for example butter, yogurt, white cheese or chocolate, according to which the product to be frozen is dropped into a bath of liquid nitrogen and is recovered in the form of solid pellets.
The articles which are introduced into the bath of cryogenic liquid are normally at room temperature or in any case, at a temperature which is much higher than that of the cryogenic liquid.
For this reason, a phenomenon of calefaction takes place. Thus, as soon as they come into contact with the cryogenic liquid, these articles heat the latter at the level of the point of contact, which contributes to convert the cryogenic liquid into gaseous state. This gas forms a layer which surrounds the article and then constitutes a slowing of the heat transfer between the article and the cryogenic liquid. This phenomenon produces a longer heat transfer between the article and the bath of cryogenic liquid and therefore it slows down the freezing of the article. Moreover, heat transfer between the article and cryogenic liquids is not ensured homogeneously, which may cause the frozen articles to be fragile.
Patent application WO 90/06693 describes a process for freezing the surface of an article according to which said article is disposed on the cooled surface of a metallic plate, which is provided with a vibratory movement. The surface of the plate is cooled by passage under the plate of a cryogenic liquid. With this type of process, the article to be frozen is not in direct contact with the cryogenic liquid and therefore does not suffer from the disadvantages mentioned above. However, it has also been observed that this type of process may also result in disadvantages for example during the treatment of pasty products such as balls of mashed vegetables or liquids. Thus, these articles adhere to the surface of the plate until their surface has been sufficiently cooled, which induces a period of latency, delaying the treatment of the articles upstream of the production line. This period of latency therefore results in a sometimes important lowering of productivity.